The cause of plagiotropy in maritime shore plants 



13 



3. The cause of the plagiotropy. 



Field observations show clearly that the moisture condition of the substratum 

 is of no decisive consequence in bringing about the plagiotropic mode of growth 

 of shore plants. One finds plagiotropy just as pronounced on a moist seaweed- 

 bank as on a hot, dry beach, provided that the first-mentioned locahty hes in as 

 exposed a situation as the latter. When working with flower-pot cultures of the 

 above-mentioned species of the genus Atriplex it was accordingl}' found that the 

 horizontal position of the above-ground organs was taken up irrespective of the 

 nature of the substratum used, whether dry or moist, pure sand, seaweed or garden 

 earth. Thus, hydrotropic factors cannot be responsible for the prostrate growth. 



Some of the previous theories as to the cause of the plagiotropy are based on 

 the assumption that the temperature of the substratum is the decisive factor. A 

 comparison between the plagiotropic plants on the hot, dry beach and on the moist 

 seaweed-bank, and similar observations in the field do not tend to strengthen this 

 behef. However, special experiments in the laboratory were necessary to decide 

 the point. Data on some of these experiments with flower-pot cultures of Atriplex 

 prostrattmi Bough have been brought together in Table I. The pots were heated to the 

 temperature indicated in the table by means of alcohol lamps placed under the 

 pots. Pots 1 and 2 were placed in very diffuse light in the laboratory, 3 and 4 

 in a dark chamber. The numerals given in pairs indicate the magnitude of the 

 angles formed by the first internode (counted from the apex) on the uppermost 

 pair of branches with the horizon. The last column gives the magnitude of the 

 deviation from the original position. A -|-sign always indicates an upward move- 

 ment of the branch, a — sign a downward movement. 



Table I. Atriplex prostratum. 





Time of 

 experim. 



Temp. C 



Original 

 position 



Final 

 position 



Deviation 



1 



2 



2 hours 



24»— 24,6» 



15, 32 



17, 33 



+ 2, +1 



2 » 



27»— 32» 



25, 19 



26, 21 



+ 1, +2 



3 





30»— 35» 



33, 53 



57, 75 



+ 24, + 22 



4 



5 » 



37»— 42» 



76, 77 



80, 79 



+ 4, +2 



These experiments show that liigh temperature does not induce any downward 

 movements of the branches. On the contrary, a rising movement of up to 24" re- 

 sulted. The great differences in the magnitude of the deviation in the different plants 

 may be interpreted in different ways, but the main point, the independence of the 

 downward movements of high temperature, is clearly brought out. 



As to the effect of heliotropism, it was found that the shoots of Atriplex and 

 Leontodon autumnalis were positively heliotropic when exposed to direct sunlight. 



